An explanation will be made by taking an example of a pattern formation method using an electron beam resist (a resist responsive to an electron beam). Namely, the method of forming a pattern by an electron beam resist ordinarily includes the steps of coating a resist on a metal thin film or a semiconductor substrate, writing a desired pattern by irradiating an electron beam to the resist film, developing the resist pattern thereafter to etch the metal thin film or the semiconductor substrate using the resist pattern as a mask, and exfoliating the useless resist. A further detailed explanation will be made by taking an example of a method of forming a pattern. Namely, a film of chromium or a film of chromium and chromium oxide is formed on a transparent glass substrate by vacuum deposition or sputtering to coat an electron beam resist on the film to write a pattern on the resist while scanning a circular electron beam in accordance with writing pattern data. Then, the resist is developed so that the written pattern is provided thereafter to selectively etch the chromium film by an etchant or etching liquid using the pattern of the resist as a mask, thus to form a chromium pattern. Finally, the unnecessary resist film is exfoliated to form a mask pattern.
Many electron beam resists are comprised of a fluoride system. While such resists have a high sensitivity with respect to the electron beam, they are disadvantageous in strong hydrophobic property (an angle in contact with water is substantially equal to 90 degrees). For this reason, when a spray wet etching is implemented using the resist pattern as a mask, the etchant is repelled on the surface of the resist because of poor wetness, resulting in water drops. Under this state, etching proceeds, leading to various drawbacks.
Namely, FIG. 8 is an explanatory view showing, in a model form, an example as to how such a spray wet etching is carried out. In FIG. 8, reference numeral 1 denotes a nozzle for an etchant. An etchant 2 is ejected in a spray manner from the nozzle 1. The sprayed etchant 2 falls on resists 3a of a mask substrate 3. This resist 3 is patterned, e.g., as shown in FIG. 9A. The resists 3 thus patterned are provided on a glass substrate 6 through a chromium thin film 5. When the etchant 2 falls on such resists 3a as a spray, the etchant takes the form of drops 2a resulting from the hydrophobic property of the resist 3a. As a result, not only development of etching becomes unevenness within the plane, but also there occur portions 5a which are not in contact with the etchant (2a) of the fine chromium exposed portion as shown in FIG. 9A, so poor wetness (the etchant does not come into contact the surface of the chromium thin film 5 because the resist 3a is in a repelled state) is apt to take place. When the resist pattern is a pattern having a small ratio of exposed portion of chromium thin film 5, or when the resist pattern is a fine and complicated pattern, such a phenomenon will be apt to occur. In addition, once a poor wetness occurs during etching, a chromium remainder 7 is caused to occur as shown in FIG. 9B. Namely, the film of chromium 5 is not completely etched, so it is left as the chromium remainder 7. As a result, a desired chromium pattern is not provided.
Also in the case of carrying out, e.g., etching by the immersion system into an etchant except for the above-mentioned spray etching, a difficulty similar to the above occurs.
Namely, FIG. 10 is an explanatory view showing, in a model form, an example of etching by the immersion system. In FIG. 10, reference numeral 8 is a liquid bath for etching in which etchant 2 is contained. When mask substrate 3 is immersed into the etchant 2, bubbles 4 adhere on the surface of the resist 3a, so a poor wetness will take place. Thus, it is difficult to provide a fine chromium pattern for the same reason as that in the case of FIG. 8.
As the method of preventing occurrence of the above-mentioned poor wetness, there have been carried out a method of using an etchant including a surface active agent, and a method of immersing a developed glass substrate into a surface active agent immediately before etching thereafter to carry out etching. However, even with these methods, the chromium being left unetched cannot be completely prevented to result in the occurrence of the chromium remainder 7.